A beat signal generating device including first and second monomode lasers for generating radiation of a first and second wavelengths respectively; a first and a second output port; a phase modulating unit for modifying both the phase of radiation generated by both the first laser and second laser, wherein radiation generated by the first laser is transmitted through the second laser and superposed with the radiation generated by the second laser at the second output port, and the radiation generated by the second laser is transmitted through the first laser and superposed with the radiation generated by the first laser at the first output port, such that a first beat signal will be emitted at the first output port and a second beat signal will be emitted at the second output port, wherein the phase between the beat signals can be adjusted by means of the phase modulating unit.